[0001] This invention relates to an electrosurgical generator for supplying radio frequency
(RF) power to an electrosurgical instrument, and primarily to a generator having a
series-resonant output network.
[0002] Conventionally, electrosurgical generators make use of a configuration comprising
a voltage source coupled to an electrosurgical instrument via a coupling capacitor
which defines a matched output impedance between 50 and 500 ohms. Such a configuration
produces a power-versus-load impedance characteristic having a power maximum at a
matched impedance, with power falling off progressively on each side of this peak.
In practice, when conducting electrosurgery, the load impedance can change over a
very wide range, resulting in unpredictable clinical effects.
[0003] To deal with this problem, it is known to provide an RF output stage capable of providing
an impedance match over a wide range. This has the disadvantage that rapid load impedance
changes can produce large output voltage excursions. An alternative approach is to
control the DC supply to the RF output stage in response to feedback signals in order
that the delivered power is virtually continuous. This may be done by adjusting the
power supply DC voltage or by maintaining the supplied DC power constant. These techniques
lead to a power versus load impedance characteristic which is virtually flat over
a range of impedances, but one limitation is that it is difficult to control the delivery
of energy when initiating tissue cutting or vaporisation (as opposed to tissue coagulation).
To cut or vaporise tissue using radio frequency power, the initial low impedance load
presented by the tissue or surrounding fluid needs to be brought to a higher impedance
in order to strike an arc. Delivering too much energy can result in bums adjacent
the operative site, excessive smoke, or instrument failure. Delivering too little
energy causes a significant delay and can result in unwanted tissue coagulation.
[0004] It is also known to use an electrosurgical generator to supply a bipolar electrosurgical
instrument with pulsed electrosurgical power at very high voltages, e.g. in the region
of 1 kilovolt peak-to-peak when removing tissue at an operation site immersed in a
conductive liquid, such as saline. The instrument may have an active electrode located
at its extreme end to be brought adjacent to or into contact with tissue to be treated,
and a return electrode set back from the active electrode and having a fluid contact
surface for making an electrical connection with the conductive liquid. To achieve
tissue removal, the conductive liquid surrounding the active electrode is vaporised
to cause arcing at the electrode. The high voltages used to achieve tissue cutting
or vaporisation under varying load impedance conditions are particularly demanding
of the generator when the instrument experiences a low load impedance. Indeed, as
stated above, under such conditions it is difficult reliably to initiate arcing without
unwanted effects. Steps have been taken to increase power density at the active electrode
and, hence, improve the reliability with which arcing is started, by reducing the
size of the electrode and by roughening its surface, e.g. by applying an oxide layer.
The latter technique has the effect of trapping vapour in the irregularities in the
surface as a means of increasing power density.
[0005] It has been found that operation of such instruments at high voltages tends to cause
erosion of the active electrode. The rate of erosion increases as the supply voltage
is increased, and is also exacerbated by reducing the size of the electrode and providing
a roughened surface, as just mentioned.
[0006] EPA 1053720 discloses a generator for generating high electrosurgical voltages.
[0007] US-A-6093186 describes an electrosurgical generator having an RF output stage and
a series resonant output network coupled between an RF power device and a pair of
output lines.
[0008] According to a first aspect of the present invention, an electrosurgical generator
for supplying RF power to an electrosurgical instrument comprises an RF output stage
having a least one RF power device, at least one pair of output lines for delivering
RF power to the instrument, and a series-resonant output network coupled between the
RF power device and the said pair of output lines, characterised in that the output
impedance of the output stage at the output lines is less than 200/√P ohms, where
P is the maximum continuous RF output power of the generator in watts, and in that
the generator further comprises protection circuitry responsive to a predetermined
electrical condition indicative of an output current overload substantially to interrupt
the RF power supplied to the output network.. When the generator is configured for
wet field surgery, e.g. for use with the electrode or electrodes of the instrument
immersed in a conductive fluid such as saline, the maximum continuous power is preferably
in the region of 300W to 400W. Accordingly, if the maximum output power is 400W, the
output impedance is less than 10 ohms. Dry field electrosurgery, i.e. with the electrode
or electrodes not normally immersed, requires less RF output power. In this case,
the generator may be configured such that the maximum continuous RF output power is
in the region of 16W, in which case that the output impedance is then less than 50
ohms. In both such cases, the figures are obtained when operating with an output voltage
for cutting or vaporising tissue, i.e. at least 300V peak. The output impedance is
preferably less than 100/√
P ohms, which yields maximum output impedance values of 5 ohms and 25 ohms at the above
power outputs.
[0009] It will be understood that when the RF output of the generator is pulsed, i.e. when
RF energy is supplied to a load in bursts, generally as an RF sine wave, the maximum
continuous power is the average power measured over several such bursts.
[0010] The protection circuitry is preferably responsive to application of a short circuit
across the output lines, the speed of response of the protection circuitry being sufficiently
fast to disable the RF power device before the current passing therethrough rises
to a rated maximum current as a result of the short circuit.
[0011] In the generator described in this specification the RF output stage has a series-resonant
output network configured such that the maximum rate of rise of the output current
at the output lines is less than (√
P)/4 amps per microsecond,
P being as defined above. Accordingly, for a typical maximum continuous RF output power
of 400W for wet field electrosurgery, the maximum rate of rise of the output current
amplitude, generally when the output lines are short-circuited at the maximum power
setting of the generator, is less than 5A/µs. With
P at a typical value of 16W for dry field electrosurgery, the rate of rise of the output
current amplitude is less than 1A/µs.
[0012] The protection circuitry is responsive to short-circuiting with sufficient speed
that the supply of RF power to the output network is cut off within a time period
corresponding to no more than 20 cycles of the delivered RF power. The protection
circuitry is preferably much faster, e.g. being operable to interrupt power delivery
within 3 cycles or even 1 cycle. The effect of the series-resonant output network
is to delay the build up of current in a fault condition such as when a very low impedance
or a short circuit appears across the output lines. The applicants have found that
an impedance transition from open to short circuit results in an effective short circuit
across the RF power device only after several RF cycles. By arranging for the protection
circuitry to respond quickly, the output stage can be disabled before that happens.
[0013] The use of an RF output stage with a relatively low output impedance means that the
RF voltage output is substantially directly related to the DC supply voltage applied
to the output stage (specifically to the RF power device or devices which it contains).
In the preferred embodiment of the invention, each RF power device is operated in
a switching mode with the result that a square wave output is applied to the series-resonant
output network. The RMS voltage available at the output lines is directly proportional
to the supply voltage. It follows that the maximum peak-to-peak output voltage is
determined by the DC supply voltage and dynamic feedback to control output voltage
is, as a result, not required in this embodiment.
[0014] The protection circuitry is preferably capable of disabling the output stage within
one-and-a-half RF periods after onset of the above-mentioned predetermined electrical
condition. Preferably, the predetermined electrical condition is indicative of an
instantaneous current in the output stage exceeding a predetermined level, and the
speed of response of the protection circuitry is such that the breaching of the predetermined
level by the instantaneous current is detected during the same RF cycle that it occurs.
Such detection may be performed by current sensing circuitry including a pick-up arrangement,
which is typically a current transformer, coupled in series between the RF power device
or devices and the series-resonant output network, and a comparator having a first
input coupled to the pick-up arrangement (e.g. to the secondary winding of the transformer)
and a second input coupled to a reference level source. The reference level source
may be a voltage representation of the instantaneous current, i.e. substantially without
filtering, in order to cause a change of state of the comparator output within the
same RF half-cycle that the threshold is first exceeded, or within the subsequent
half-cycle, depending on whether or not full wave rectification is applied ahead of
the comparator. The predetermined instantaneous output level is preferably at least
5A for wet field electrosurgery, and typically 15A. The output of the comparator is
coupled to disabling circuitry to disable the power device or devices when the comparator
output changes state in response to the instantaneous current sensed by the pick-up
arrangement exceeding the predetermined level as set by the reference source. The
current shut-down aspect of the protection circuitry is not limited by impedance.
[0015] Generally, it is necessary only to interrupt power delivery for a short time. Consequently
the protection circuitry includes a monostable stage and is operable in response to
detection of the predetermined condition to disable the power device for a limited
period determined by a time constant of the monostable stage which is typically less
than 20 cycles of the operating frequency of the generator.
[0016] Preferably, the generator has an RF source coupled to the power device or devices,
the source including an oscillator defining the operating frequency of the generator.
The series resonant output network is tuned to this operating frequency. Generally,
the source is arranged such that the operating frequency is substantially constant
(e.g. during any given treatment cycle).
[0017] The preferred generator is arranged such that, for a given user setting, the RMS
RF output voltage is substantially within a load impedance range of from 600/√
P ohms to 1000 ohms, where
P is as defined above. Thus, for instance, the RMS RF output voltage constant during
each burst of RF energy is maintained to within 20 percent of a maximum value. This
can be achieved partly as a result of the series-resonant configuration of the output
network.
[0018] To maintain the constant peak output voltage at low impedances, according to a particular
preferred feature of the invention, the RF power supply to the output stage includes
a charge-storing element, preferably a capacitance in excess of 1mF, the output devices
being pulsed by a pulsing circuit so that they supply RF energy in bursts with the
timing of the bursts, particularly the termination of each burst, being controlled
in response to the output of a voltage sensing circuit coupled to the capacitance.
The DC power supply voltage to the output stage is preferably 100V or greater. To
avoid substantial decay of the supply voltage, the voltage sensing and pulsing circuits
are arranged to terminate the individual pulses of RF energy when the sensed voltage
falls below a predetermined level, typically set such that pulse termination occurs
when the voltage falls by a predetermined percentage value of between 5 percent and
20 percent which, typically, corresponds to the peak RF voltage delivered at the output
lines falling to a value between 25V and 100V below its starting value for the respective
pulse. The RF energy delivered during each pulse is typically 60 joules for wet field
electrosurgery and 2 joules for dry field electrosurgery. Peak power typically reaches
at least 1kW, and preferably 4kW.
[0019] The very high peak power capability of the preferred wet field generator (in excess
of 1kW) allows the impedance transition occurring at the start of a tissue cutting
or vaporisation cycle to be completed very quickly since only voltages in excess of
those required for arcing are delivered. This significantly reduces the delay and
the unwanted coagulation effects of some prior art generators. The substantially constant
voltage delivery leads to cutting or vaporisation occurring at consistent rates, regardless
of changes in tissue type or engagement.
[0020] The RF output stage of the preferred generator, when the latter is configured for
wet field electrosurgery, has an output impedance at the output lines of less than
10 ohms. When the generator is configured for cutting or vaporising tissue in dry
field electrosurgery the output impedance of the output stage at the output lines
is less than 50 ohms.
[0021] The generator described in this specification is configured to be capable of maintaining
a peak output voltage of at least 300V over a load impedance range of from 600/√
P ohms to 1000 ohms, where
P is the rated output power in watts. The rated output power is as defined in the International
Electrotechnical Commission standard, IEC 60601-2-2. It has a power supply stage coupled
to the RF output stage, the power supply stage having an energy storage capacitor
capable of storing between 3 percent and 30 percent of the maximum continuous power
P (in watts) of the generator in joules. Typically, the energy delivery per pulse (in
joules) is between 1 percent and 10 percent of the maximum continuous RF output power
(in watts).
[0022] In the preferred generator the pulsing circuit pulses the delivered RF power in such
a way that the crest factor of the voltage developed across the output lines increases
as the load impedance presented to the output lines decreases whilst the peak output
voltage during pulses is maintained at a value greater than 300V. For wet field electrosurgery,
the output impedance of the output stage is preferably less than 10 ohms and the crest
factor varies by a ratio of at least 2:1 over a load impedance range of from 600/√
P to 1000 ohms (typically from 10 ohms to 1000 ohms). For dry field electrosurgery,
the output impedance figure is less than 50 ohms, and the crest factor varies by a
ratio of at least 2:1 over a load impedance range of 600/√
P to 50 kilohms (typically from 50 ohms to 50 kilohms).
[0023] By "crest factor" we mean the ratio of the peak voltage to the RMS voltage. In the
case of a pulsed output waveform, the measurement is conducted over plurality of pulses.
[0024] The preferred generator comprises a source of radio frequency (RF) energy, an active
output terminal, a return output terminal, a DC isolation capacitance between the
source and the active output terminal, and a pulsing circuit for the source, wherein
the source and the pulsing circuit are arranged to generate a pulsed RF output signal
at the output terminals, which signal has a peak current of at least 1A, a simultaneous
peak voltage of at least 300V, a modulation rate of between 5Hz and 2kHz, and a pulse
length of between 100µs and 5ms. The signal has a peak current of at least 3A.
[0025] With such a generator it is possible to start arcing even under conditions of relatively
low load impedance. Once an arc is established, the load impedance tends to rise,
to the extent that the arcing can be maintained using a continuous RF output waveform.
Improved power density is available at the active electrode for vaporisation, whilst
reducing electrode erosion.
[0026] The length of the pulses is preferably between 0.5ms and 5ms, the pulse duty cycle
typically being between 1% and 20% and, more preferably, between 2% and 10%.
[0027] The preferred generator in accordance with the invention has a resonant output network
and is operable to generate, e.g. during at least an initial part of a treatment period,
a peak power of at least one kilowatt, and typically at least 3 or 4 kilowatts. Improvements
in electrode erosion performance can be achieved by providing means in the generator
for limiting the output voltage to a value in the region of 900V to 1100V peak-to-peak.
[0028] In the preferred generator, the source and the pulsing circuit are arranged to generate,
in an initial period, a pulsed RF output signal at the output terminals, which signal
has a peak current of at least 1A, a simultaneous peak voltage of at least 300V, a
modulation rate of between 5Hz and 2kHz, and a pulse length of between 100µs and 5ms,
and, in a subsequent period, to generate a constant power RF output signal at the
output terminals.
[0029] Different ways of causing the generator to end the above-mentioned initial period
of operation and begin the so-called subsequent period are feasible. One generator
embodiment is arranged such that the switchover from the initial period to the subsequent
period occurs automatically at a predetermined time interval after the beginning of
the initial period. In an alternative embodiment, the generator has means for monitoring,
in use of the generator, the load impedance between the active and return output terminals,
and is arranged to cause switchover to the subsequent period when the magnitude of
the output impedance increases by a predetermined factor, typically between 5 and
20, and preferably 10, or when it exceeds a predefined threshold.
[0030] The preferred generator uses a third switching-over technique involving the charge-storing
element mentioned above. In this case, the source of RF energy includes an RF output
stage, and the generator has a power supply including the charge-storing element such
as a large capacitor for supplying power to the output stage. When the treatment period
includes an initial period and a subsequent period, as described above, the capacitor
is used to supply power at least during the initial period. Associated with the charge-storing
element is a voltage-sensing circuit for sensing the voltage supplied to the output
stage by the charge-storing element, the generator being arranged such that treatment
ends or the subsequent period begins in response to the supply voltage as sensed by
the voltage-sensing circuit reaching a predetermined voltage threshold. Indeed, it
is possible to control the length and timing of individual pulses using the same voltage-sensing
circuit. In this case, the voltage-sensing circuit forms part of the above-mentioned
pulsing circuit and the timing of at least the beginnings of the pulses produced by
the output stage during the initial period being determined in response to the supply
voltage reaching the above-mentioned voltage threshold. It is possible to arrange
for both the leading and trailing edges of the pulses produced by the output stage
to be determined by the supply voltage respectively falling below and exceeding the
respective voltage thresholds.
[0031] The charge-storing capacitance is preferably at least 1000µF and advantageously has
a capacity in excess of 5J.
[0032] As already stated, the preferred generator has a tuned output. Indeed, good results
have been obtained using a generator with a resonant output network, the load curve
of the generator (i.e. the curve plotting delivered power versus load impedance) having
a peak at a load impedance below 50 ohms. Delivery of peak power levels into low load
impedances is aided by forming the output network as a series-resonant network comprising
the series combination of an inductance and a capacitance, the output of the network
being taken across the capacitance. The output may be taken to all output terminal
of the generator via a coupling capacitor and, optionally, a step-up transformer from
a node between the inductance and the capacitance of the series combination. Whilst
it is possible, instead, to take the output from across the inductance, taking it
across the capacitor has the advantage of reducing switching transients. As a further
alternative, the generator may have its output terminals connected to the resonant
output network so that, effectively, when a load is connected to the terminals it
is connected as an impedance in series with the inductance and capacitance forming
the resonant combination, e.g. between the inductance and the capacitance.
[0033] The resonant output network typically provides a source impedance at the output terminals
in the range of from 50 ohms to 500 ohms.
[0034] Not least because the resonant frequency of the output network can vary with load
impedance as a result of coupling capacitance, the RF source may include a variable
frequency RF oscillator, the output frequency advantageously being limited to a maximum
value below the resonant frequency of the output network when connected to a matching
load impedance, i.e. a load impedance equal to its source impedance.
[0035] The generator may be combined with a bipolar electrosurgical instrument to form an
electrosurgical system, the instrument having at least an active electrode coupled
to the active output terminal of the generator and a return electrode coupled to a
generator return output terminal. The invention has particular application to an electrosurgery
system in which the bipolar electrosurgical instrument has an active electrode formed
as a conductive, preferably U-shaped loop. Such a loop is often used for excising
tissue samples but places particular demands on the generator in terms of achieving
saline vaporisation and arcing.
[0036] A preferred electrosurgery system comprises a generator having a source of radio
frequency (RF) energy and, coupled to the generator, a bipolar electrosurgical instrument
having an electrode assembly with at least a pair of electrodes, wherein the generator
is adapted to deliver RF energy to the electrode assembly in an initial period as
a pulse modulated RF signal which, in use with the pair of electrodes, has a peak
current of at least 1A, a simultaneous peak voltage of at least 300V, a modulation
rate of between 5Hz and 2kHz, and a pulse length of between 100µs and 5ms.
[0037] Again, the system may be adapted to deliver RF energy to the electrode assembly,
in an initial period, as a pulse modulated RF signal which, in use with the pair of
electrodes, has a peak current of at least 1A, a simultaneous peak voltage of at least
300V, a modulation rate of between 5Hz and 2kHz, and a pulse length of between 100µs
and 5ms, and to deliver RF energy to the electrode assembly in a subsequent period
as a continuous power RF signal. The peak current is preferably at least 3A.
[0038] The invention will now be described by way of example with reference to the drawings
in which:-
Figure 1 is a general diagram showing an electrosurgery system including a generator
in accordance with the invention and a bipolar electrosurgical instrument;
Figures 2A and 2B are respectively perspective and side views of a loop electrode
assembly forming part of the bipolar instrument shown in Figure 1;
Figure 3 is a block diagram illustrating the main components of the generator;
Figure 4 is a simplified circuit diagram of an RF output stage forming part of the
generator;
Figure 5 is an illustrative load curve for the generator of Figure 1;
Figure 6 is a more detailed circuit diagram of the RF output stage;
Figure 7 is a block diagram of an alternative electrosurgical generator in accordance
with the invention;
Figure 8 is a circuit diagram of a resonant output network of the alternative generator;
and
Figure 9 is the load curve of the generator of Figure 7.
[0039] Referring to Figure 1, a generator 10 has an output socket 10S providing a radio
frequency (RF) output for an electrosurgical instrument in the form of an endoscope
attachment 12 via a connection cord 14. Activation of the generator may be performed
from the instrument 12 via a control connection in cord 14 or by means of a footswitch
unit 16, as shown, connected separately to the rear of the generator 10 by a footswitch
connection cord 18. In the illustrated embodiment, the footswitch unit 16 has two
footswitches 16A and 18B for selecting a coagulation mode and a cutting mode of the
generator respectively. The generator front panel has push buttons 20 and 22 for respectively
setting coagulation and cutting power levels, which are indicated in a display 24.
Push buttons 26 are provided as alternative means for selection between coagulation
and cutting modes. The instrument 12 has a detachable loop electrode assembly 28 with
a dual electrode structure and is intended for use in a saline field.
[0040] The instrument 12 has a detachable loop electrode assembly 28 with a dual electrode
structure and intended for use in a saline field. Figures 2A and 2B are enlarged views
of the distal end of the electrode assembly 28. At its extreme distal end the assembly
has a U-shaped loop electrode 30 depending from a pair of electrode assembly arms
32 which are mounted side-by-side in a clip 34 intended to be snapped onto an endoscope.
The loop electrode 30 is an active electrode. Each of the arms 32 is formed as a coaxial
cable, the exposed conductive outer shield of which, in each case, forms a return
electrode 36. In operation immersed in a saline field, the loop electrode 30 is typically
used for excising tissue samples, the electrosurgical voltage developed between the
loop electrode 12A and fluid contacting surfaces of the return electrodes 36 promoting
vaporisation of the surrounding saline liquid at the loop electrode 30, and arcing
through the vapour envelope so formed.
[0041] The loop electrode 30 comprises a composite molybdenum rhenium wire with an oxide
coating to promote increased impedance in the electrode/fluid interface and, as a
result, to increase power density at the surface of the electrode.
[0042] The width of the loop is typically in the range of 2.5mm to 4mm and the wire typically
has a diameter in the range of 0.20 to 0.35mm.
[0043] This loop electrode assembly places particular demands on the generator in terms
of starting vaporisation and arc formation.
[0044] Efforts to improve the starting of the arc (the "firing up") of this electrode assembly
by reducing the wire diameter and forming oxide layers have tended to increase the
rate of erosion or resulted in the loop being mechanically flimsy.
[0045] It should be noted that generators in accordance with the invention not limited to
use with a loop electrode assembly, nor to use in wet field surgery.
[0046] The generator will now be described in more detail with reference to Figure 3. It
has an RF source in the form of an oscillator 40 which is connectible to an RF output
stage 42. The output stage 42 comprises a mosfet power bridge forming part of a power
mosfet and driver circuit 44, a current sensing element 46 and a resonant output network
48. The oscillator 40 is configured to operate at a substantially constant RF frequency
and the output network 48 is tuned to that frequency. In general terms, the RF source
coupled to the RF power device or devices defines the operating frequency of the generator,
and the output network (which, as will be described below, is series-resonant) is
tuned to the operating frequency. In this embodiment of the invention the operating
frequency is substantially constant.
[0047] Power to the RF output stage 42, or, more specifically, to the power mosfets, is
supplied from a DC power supply 50 via a supply rail 58. A 4.7mF reservoir capacitor
60 is connected between the supply rail 58 and ground. The voltage on the supply rail
58 is sensed by a voltage sensing circuit 62 which controls a first transmission gate
64 connected in series between the RF oscillator 40 and driver devices in the power
mosfet and driver circuit 44.
[0048] The current sensing element 46 in the output stage 42 is a series-connected current
transformer, the secondary winding of which is coupled to a first input of a comparator
66 which also receives on the other of its inputs a reference signal from a reference
input 68. The output of the comparator controls a monostable 70 which, in turn, controls
a second transmission gate 72 coupled in series with the gate 64 in the path between
the oscillator 40 and the drivers in the power mosfet and driver circuit 44. The output
network 48 supplies RF power to an output termination 74 which, in practice, is a
pair of output lines, as will be described hereinafter. Operation of the generator
is normally pulsed insofar as RF energy is supplied to the output lines 74 in bursts
controlled by the combination of the voltage sensing circuit 62 and gate 64 which
operates as part of a pulsing circuit. When the generator is activated, the voltage
on the supply rail 58 tends to fall, at least when the load impedance coupled across
output lines 74 is relatively low, owing to the discharge of reservoir capacitor 60.
When the DC supply voltage on the supply rail 58 falls to a preset value, the output
of the voltage sensing circuit 62 changes state and transmission gate 64 is driven
to its open circuit condition, thereby disabling the power mosfet and driver circuit
44. The reservoir capacitor 60 then recharges and the voltage sensing circuit 62 causes
the gate 64 to reconnect the oscillator 40 when the supply rail voltage reaches a
second, higher present value. In this way it is possible to control the amount of
energy delivered in each pulse.
[0049] The current sensing element 46, the comparator 66, the monostable 70 and the second
transmission gate 72 act together as a protection circuit to protect the mosfet power
devices in circuit 44 against excessive current drain caused, for instance, by a short
circuit across the output lines 74. The storage of energy in output network 48 delays
the transfer of the short circuit across the output lines 74 to the power devices
in the mosfet and driver circuit 44.
[0050] The electrical circuit condition sensed by the current sensing element 46 and the
comparator 66 is the current flowing between the power mosfets in circuit 44 and the
output network 48 rising to a level which could be indicative of a short circuit having
been applied across the output lines 74. When the current reaches a preset current
level, as detected by the comparator 66, the comparator output changes state and the
monostable 70 causes the second transmission gate 72 to become open circuit, disabling
the power mosfets and driver stage 44. The monostable time constant is typically set
to 0.5 seconds or less, which allows a warning signal to be generated for alerting
the user. However, owing to energy storage in the series-resonant circuit, it is possible
to protect the RF power devices with a monostable time constant of about 20 RF cycles
at an operating frequency of 400kHz.
[0051] The configuration of the output stage 42 is shown in principle in the simplified
circuit diagram of Figure 4. Referring to Figure 4, the power mosfet and driver stage
44 shown in Figure 3 has a power mosfet bridge comprising a first push-pull pair of
FET power devices Q1, Q2 and a second power FET device push-pull pair Q3, Q4, each
pair having a respective output node which, when the pairs are driven 180° out of
phase, produces a square wave at the frequency of the oscillator 40 (Figure 2) at
the input to the series resonant output network 48. Each pair of power mosfets Q1,
Q2; Q3, Q4 is coupled between the supply rail 58 and ground. Accordingly, since each
of the mosfets is a virtual short circuit when driven "on", the voltage applied to
the output network 48 swings virtually between ground and the supply rail voltages.
The reservoir capacitor 60 shown in Figure 3 is, of course, connected across the respective
power mosfet pairs, as shown in Figure 4.
[0052] The output network is series-resonant in that an inductor L1 and a resonating capacitor
C1 are coupled in series between the output nodes 76, 78 of the first and second power
mosfet pairs respectively. In this embodiment, the load resistance R
L constituted in practice by an electrosurgical instrument coupled between the output
lines 74, and the tissue and/or fluid present across its electrode assembly, is connected
in series between inductor L1 and capacitor C1. As explained above, the series-resonant
tuned circuit formed by inductor L1 and capacitor C1 acts as an energy storing device
which delays the current build-up between the nodes of the power mosfet bridge Q1-Q4
should the load resistance R
L drop to a very low value. Another feature of this resonant arrangement is that it
is a low impedance at one frequency only, which means that the delivered output signal
consists almost exclusively of the fundamental component of the waveform produced
by the power mosfets, conditional, of course, upon the frequency of resonance of the
network 48 being the same as that of the operating frequency of the oscillator stage
40 (Figure 3).
[0053] One of the characteristics given to the generator by the output configuration described
above with reference to Figure 4 is that, during each burst or pulse of RF energy
it has an approximately constant voltage load curve, as shown by the power-versus-load
impedance load curve shown in Figure 5. This characteristic is particularly suitable
for cutting or vaporisation of tissue since it provides the high power required at
low impedance without voltage overshoot. The low output impedance and high current
required are provided by the direct coupling of the power mosfets to the supply rail
and ground, and by the reservoir capacitor 60, even if a step-up transformer is coupled
between the series-resonant elements L1, C1 and the output lines 74. It is possible,
using this configuration, to keep the output impedance of the generator at the output
lines 74 to 2 ohms or less. The implications which this has for peak current delivery
in a fault condition leads to the need for a protection circuit such as that referred
to above.
[0054] The RF output stage 42 is shown in more detail in Figure 6. As shown in Figure 6,
the current sensing element 46 is a current transformer, coupled in series between
one of the output nodes 76, 78 of the power mosfet bridge and one of the components
L1, C1 of the series resonant output network, in this case between node 76 and the
inductor L1. In this preferred generator, the normal DC supply voltage on supply rail
58 is about 120V. To strike an arc for the purpose of performing tissue cutting or
vaporisation, a peak voltage in excess of 380V may be required. Accordingly, and for
isolation purposes, the RF output network 48 includes a step-up isolating transformer
TR1 to lift the peak output voltage to the region of 500V peak. The primary winding
of the transformer TR1 has a tuning capacitor C2 coupled across it to yield a parallel-resonant
circuit tuned to the operating frequency to act as a shunt-connected trap. This improves
the rejection of harmonics in the power signal supplied to the output lines 74., particularly
when the output impedance is high, with the consequent benefit of reduced RFI (RF
interference).
[0055] DC blocking is provided by a coupling capacitor C3 between the transformer TR1 secondary
winding and one of the output lines 74.
[0056] The actual resonant frequency of the output network 48 is the result of several elements,
these being (1) the main tuning elements represented by the lumped inductance L1 and
the tuning capacitor C1, (2) the transformer leakage inductance and cross-coupling
capacitance, (3) the DC blocking capacitance, C3, and (4) the inductive and capacitive
loading of the connecting cable (not shown) between the output lines 74 and the electrosurgical
instrument itself. The delay in the current build-up in a fault condition is due to
the energy levels in this tuned arrangement. At resonance, this arrangement has a
finite loss that may be represented by a series resistance, albeit a very small one.
Dynamically, however, the energy levels in the resonant output network cannot be changed
instantly. An impedance transition from an open to short circuit only presents a short
circuit to the switching stage after several RF cycles at the operating frequency.
The comparator 66 shown in Figure 3 is capable of detecting such an impedance transition
within 1 to 1.5 cycles of the transition beginning at the output lines 74. This rapid
response, as well as allowing the power mosfet and driver circuit 44 to be shut down
before damage occurs, has the effect that the amount of energy delivered during a
short circuit fault is very small.
[0057] Referring again to Figure 3 and, in particular, the voltage sensing and output stage
pulsing circuits 62, 64, the very high peak powers which are achievable with the output
stage described above with reference to Figures 5 and 6 have the benefit that, during
power delivery into a low impedance, the voltage across the reservoir capacitor 60
decreases progressively after the instant of generator activation. The capacitor value
is chosen to be sufficiently large to ensure that the low to high load impedance transition
occurring at the start of a tissue cutting or vaporisation cycle can be produced in
a single burst of RF energy. Typically, the amount of energy delivered during the
initial burst is about 1 joule in a dry environment and between 10 to 20 joules in
a wet field environment. The actual energy in the RF pulses or bursts is controlled
by the threshold or thresholds set in the voltage sensing circuit 62, specifically
by the difference in supply voltage between pulse initiation and pulse termination.
Since the output stage has a very low output impedance, this difference voltage is
apparent as a change in delivered RF voltage at the output. The capacitor 60 is, therefore,
made sufficiently large (in this embodiment 4.7mF) that the change in voltage represents
only a minor proportion of the absolute voltage at the output. Thus, if the delivered
output voltage is a sine wave with a peak voltage of 500V, the supply voltage on supply
rail 58, the size of the capacitor 60 and the transformer TR1 step-up ratio are chosen
such that the output voltage drops by no more than 100V peak (20 percent) during an
RF burst. In this preferred embodiment, the output voltage drop is about 50V peak
or 10 percent.
[0058] One of the effects of preventing the supply of lower voltages to the output is that,
in a tissue cutting or vaporisation tissue cycle, the voltage is not allowed to drop
to a level at which undesirable coagulation effects occur.
[0059] The preferred generator in accordance with the present invention allows the DC energy
fed to the reservoir capacitor 60 to be altered so that the time period during which
a cutting voltage is present at the output can be altered. In practice, owing to the
low output impedance of the generator, this time period is directly proportional to
the stored energy.
[0060] The control methodology, whereby RF energy bursts or pulses are controlled according
to voltage thresholds sensed across a reservoir capacitor, allows very low duty cycles
to be used, permitting tissue cutting or vaporisation at low average powers. Indeed,
it is possible to operate with less than 5 watts average power (averaged over several
capacitor charging and discharging cycles). Accordingly, the generator has uses in
low power as well as high power applications.
[0061] An alternative generator for use in the system described above with reference to
Figure 1 will now be described with reference to Figure 7. This generator has a variable
frequency RF source including a voltage controlled oscillator (VCO) 40A. In this example,
the VCO feeds a divide-by-two stage 40B which, in turn, feeds a power driver stage
44A driving an RF output stage in the form of a power bridge 44B. The power bridge
44B feeds a resonant output network 80 which delivers a generator output signal across
output terminals 74. In practice, the power driver stage 44A and the power bridge
44B can have the same configuration as the power mosfet and driver circuit 44 of the
generator described above with reference to Figure 3. The power bridge 44B takes its
DC supply from the supply rail 58 of the DC power supply 50, but the driver stage
44A has a lower voltage supply. Typical supply voltages are 180V maximum for the power
bridge 44B and 16.5V for the driver stage 44A.
[0062] To bring the frequency of the combination of the VCO 40A and divide-by-two stage
40B to the resonant frequency of the output network 80, the above-described components
of the RF source are coupled in a phase-locked loop including a phase sensing element
82 coupled between the power bridge 44B and the output network 80 to sense the current
phase in the input leads to the output network. This current phase signal is applied
to one input of a phase comparator 84, the other input of which receives a signal
representative of the output of the VCO 40A, derived from the output of the divide-by-two
stage 40B via a delay stage 86 which compensates for the delay to the RF signal as
it passes through the power driver and the power bridge.
[0063] As in the first-described generator, the RF output stage 44B is supplied from the
DC supply rail 58 attached to the reservoir capacitor 60, which allows large currents
to be drawn by the output stage 44B for short periods of time, i.e. currents significantly
larger than the current rating of a power supply (not shown) connected to the DC supply
rail 58. It follows that the voltage on supply rail 58 will fall during the time that
a large current is drawn. Such variations in voltage are sensed by the voltage sensing
stage 62 coupled to the rail 58. Voltage sensing circuit 62 has a control output coupled
to the first transmission gate 64 in a line 88 coupling the divided-down output of
the VCO 40A to the input of the power driver 44A.
[0064] As before, the arrangement of the voltage sensing stage 62 and the gate 64 are such
that when the voltage on supply rail 58 (the voltage supplied to the power bridge
44B) drops below a predetermined voltage threshold, the gate 64 is operated to interrupt
the signal path between the VCO and the power driver 44A. When the supply rail voltage
rises again, the gate 64 reverts to its conducting state. This may happen when the
voltage rises above the threshold mentioned above, or a second threshold voltage.
[0065] The second transmission gate 72, connected in series in the signal line 88 with the
voltage-operated gate 64, has a control input connected to the output of a 0.5 second
monostable 70 which is triggered by current sensing circuitry comprising the current
sensing element 46 in one of the input leads to the output network 80 and the comparator
66. These elements act to interrupt the signal line 88 to the power driver 44A for
0.5 seconds when the power bridge output current exceeds a predetermined threshold.
[0066] Referring to Figure 8, the resonant output network 80 comprises the series combination
of an in-line inductance L and a tank capacitor C
1. The output is taken from across the tank capacitor C
1 (which takes out switching noise) via a first coupling capacitor C
2. This first coupling capacitor C
2 couples to the output (represented by terminals 74) via a step-up matching transformer
T with a 1: 2 step-up ratio. The secondary rewinding of the transformer T couples
to the output terminals via a second coupling capacitor C
3. In this embodiment, L is about 0.47µH, the tank capacitor is about 10nF and the
two coupling capacitors C
2 and C
3 co-operate (one of them via the transformer T) to form a coupling capacitance of
about 23nF.
[0067] It will be appreciated that when the output terminals 74 are open-circuit, the resonant
frequency of the output network is determined by the series combination of L and C
1. When the output terminals 74 are shorted, the resonant frequency is determined by
the series combination of L and the network represented by C
1, C
2, C
3 and T. With the values given, the short-circuit resonant frequency is about 0.55
times the open-circuit resonant frequency.
[0068] One of the features of a series-tuned output stage is that peak power delivery inherently
occurs at extremely low and extremely high impedances. Referring to Figure 9, the
load curve of a series-tuned network (i.e. the delivered power versus load impedance)
at resonance is shown by the dotted curve A. The network 80 has minimum power delivery,
which may be regarded as the "matched condition", at a load impedance across the terminals
74 (Figure 7 and 8) of about 200 ohms. It will be noted that the part of the curve
A which has a negative slope follows a path which is approximately hyperbolic over
a major part of its length, which means that this part of the curve is of similar
shape to a constant voltage line on the graph of Figure 9.
[0069] The applicant has recognised that such a characteristic, when applied to the output
stage of an electrosurgical generator, allows output power to be maximised for a given
constant voltage limit over a range of load impedances. It has been found that erosion
of the active electrode of an electrosurgical instrument operated in a conductive
liquid increases markedly when the output voltage rises above a threshold in the region
of 900 volts to 1100 volts peak-to-peak. By arranging for the load curve of the output
network 48 to follow an approximate constant voltage curve at about 1000 volts peak-to-peak
(340 volts rms) the power delivered into a varying load impedance can be close to
the maximum theoretically achievable for that voltage.
[0070] In effect, over the range of load impedances of importance in so-called "underwater"
electrosurgery, the generator can be made to behave as a constant voltage supply.
This can be achieved with a matched output impedance much higher than the load impedance
presented by the electrode assembly shown in Figure 2A and 2B in the wetted condition,
which, for a 4mm loop is in the region of 25 ohms. This translates to a maximum power
of about 4.5kW at 340 volts rms.
[0071] The actual load curve achieved with the arrangement shown in Figures 7 and 8 is shown
by curve B in Figure 9. This deviates from the series-tuned curve A at low impedances
owing to imposition of a current limit using the current sensing stage circuitry 46,
66 monostable 70 and transmission gate 72 (Figure 7). In the present embodiment, the
current limit is set at a level of about 13 amps. The actual load curve B also deviates
from the inherent series-tuned load curve A towards the lower part of the negative-slope
portion of the curve A so that the delivered power follows a continuing negative gradient
as the load impedance rises, again mimicking a constant voltage supply. This latter
deviation is deliberate inasmuch as extreme power into a very high impedance is undesirable.
The reason for this deviation is the movement of the resonant frequency of the output
network 80, as described above, coupled with the imposition of a high-frequency limit
on the RF frequency output as will be described below. The phase comparator 84 compares
the current phase at the input to the output network 80, as sensed by the phase sensing
circuit 82 with a delayed version of the output of the divide-by-two circuit 40B which,
in turn, is fed by the VCO 40A. Accordingly, the phase and frequency of the VCO are
varied to maintain a constant phase at the input to the output network 80, subject
to the upper frequency limit mentioned above. In the absence of other influences,
therefore, the output network 80 is maintained in resonance as the load impedance
varies.
[0072] Given that the free-running frequency of the phase-locked loop is arranged to be
its maximum frequency of operation, the locking characteristics of the phase-locked
loop are such that it can be brought into a locked condition at the minimum frequency,
corresponding to minimum load impedance, sufficiently quickly to achieve resonance
in the early part of the output pulse, but not so quickly that the current limit circuit
(sensing circuitry 46, 66 monostable 70 and gate 72) fails to trip when the current
exceeds a predetermined current threshold.
[0073] If, now, the output carrier frequency is limited to a value below the frequency of
the matched load resonant condition, the delivered power will fall off as the load
impedance increases and the resonant frequency correspondingly rises. In fact, the
free-run output frequency of the phase locked loop containing the VCO 40A (Figure
7) is designed to be this maximum frequency. This ensures that the output network
always represents a higher source impedance than the impedance of the load, which
affords over-voltage protection in the event of a short.
[0074] Summarising, to achieve optimum resonant frequency, the excitation oscillator (VCO)
is phase-locked to the resonant output network. Defining the range of the VCO provides
load curve definition in that the delivered output power falls below the theoretical
maximum when the output network resonant frequency rises above the maximum frequency
of the divided down output of the VCO 40A. In other words, a match at high load impedance
is prevented by preventing the VCO from generating the higher frequencies necessary
for resonance. It also follows that, at high load impedances, the maximum output voltage
is controlled by virtue of frequency.
[0075] It will be seen from Figure 9 that the delivered output power is in excess of 1kW
over a range of load impedances corresponding to a wetted or partly wetted electrode.
Once vaporisation and arcing has been initiated, the impedance rises, and the delivered
power falls. To maintain the average output power at 200W or less, the output signal
is pulsed when the load impedance is low. It will be understood that with a peak power
in excess of 4kW, the pulse duty cycle needs to drop to a level in the region of 5%
or less. The pulse repetition rate should be between 5Hz and 2kHz, and is preferably
at least 10Hz. These figures are chosen in view of the time taken to initiate vaporisation
at the electrode surface. This means that the pulses have a maximum length of about
4 or 5ms into a low impedance requiring maximum power. Typically, the pulse length
is in the region of 1 to 2ms. While it is not essential, configuring the RF output
stage of the generator as an amplifier amplifying the output of a signal derived from
a separate oscillator, rather than having a self-oscillating output stage, is preferred
in order that full peak power can be achieved within the above-stated pulse lengths.
(In this embodiment, the output stage 44B is an amplifier configured as a power switching
bridge for high efficiency.) Should the VCO fail to operate at a frequency corresponding
to resonance of the output network 80, as may happen at the start of each pulse, excessive
output currents associated with such a mismatch are prevented since the series-tuned
output network is low impedance only at resonance.
[0076] Pulsing of the output signal can be performed in a number of ways, including simply
pulse modulating with predetermined pulse lengths and pulse repetition rates. In the
mode of operation of the alternative generator described here, the output is pulsed
only during an initial period from the commencement of treatment, the output signal
being a continuous wave (CW) signal thereafter, i.e. generally when vaporisation and
arcing have been achieved and the load impedance is in an upper range. The duration
of the initial period may be fixed or it may be determined by monitoring the load
impedance and terminating the initial period when the impedance exceeds a predetermined
value. In this embodiment, the duration of the initial period and the length and frequency
of the pulses are dynamically variable in response to delivered energy, as measured
by the supply rail voltage on supply rail 58. As has been explained above, high instantaneous
power levels are achieved only by allowing the output stage 44B to draw current from
a charge reservoir, here a large capacitance such as capacitor the 47mF capacitor
60. As charge is drawn from the capacitor 60, the supply rail voltage drops. Between
pulses, the supply rail voltage rises again. Accordingly, by using gate 64 alternately
to allow and prevent the passage of an RF signal along signal line 88 to the power
driver 44A according to the relationship between the supply voltage level and a threshold
or thresholds set in the voltage sensing circuit 62, the output of the generator can
be pulsed to achieve maximum peak delivered power whilst operating within a predetermined
average power limit. This equilibrium of power consumption and DC supply voltage is
achieved by setting the voltage thresholds so that the RF output stage is activated
when the supply rail voltage is sufficient to achieve a maximum vaporisation voltage
(e.g. 340V rms) and switched off when a lower threshold is reached. The lower threshold
defines the maximum energy per pulse and the repetition rate for a given average power
level. The initial period referred to above is terminated when the electrode has "fired-up",
in other words when vaporisation and arcing have commenced, so that the load impedance
rises and the supply rail voltage stays above the switching threshold or thresholds.
In this way it is possible to achieve vaporisation of the conductive liquid surrounding
the electrode at impedances as low as 20 ohms without unacceptable erosion of the
electrode surface.
1. An electrosurgical generator (10) for supplying radio frequency (RF) power to an electrosurgical
instrument (12) for cutting or vaporising tissue, wherein the generator comprises
an RF output stage having: at least one RF power device (Q1 - Q4), at least one pair
of output lines (74) for delivering RF power to the instrument, and a series-resonant
output network (L1, C1) coupled between the RF power device and the said pair of output
lines, characterised in that the output impedance of the output stage at the output lines is less than 200/√P ohms, where P is the maximum continuous RF output power of the generator in watts, and in that the generator further comprises protection circuitry (46, 66, 70, 72) responsive
to a predetermined electrical condition indicative of an output current overload substantially
to interrupt the RF power supplied to the output network.
2. A generator according to claim 1, characterised by protection circuitry (46, 66, 70, 72) responsive to application of a short circuit
across the output lines (74), and characterised in that the series-resonant output network (L1, C1) is such that the rate of rise of the
output current at the output lines when the short circuit is applied is less than
(√P)/4 amps per microsecond.
3. A generator according to claim 1, characterised by protection circuitry (46, 66, 70, 72) responsive to application of a short circuit
across the output lines (74), and characterised in that the protection circuitry is responsive to the said short circuit sufficiently quickly
to disable the RF power device (Q1 - Q4) before the current passing therethrough rises
to a rated maximum current as a result of the short circuit.
4. A generator according to claim 3, characterised in that the power device (Q1 - Q4) is disabled in response to the application of the short
circuit to the output lines (74), the disabling occurring in a time period corresponding
to less than 3 RF cycles of the delivered RF power.
5. A generator according to any preceding claim, characterised in that the predetermined electrical condition is indicative of an instantaneous current
in the output stage exceeding a predetermined level, and in that the speed of response of the protection circuitry (46, 66, 70, 72) is such that the
said condition is detected within the RF cycle during which the instantaneous current
exceeds the said level.
6. A generator according to any preceding claim,
characterised by:
a power supply stage (50, 60, 62) coupled to the RF output stage, the power supply
stage including a charge-storing element (60) for supplying power to the power device
or devices (Q1 - Q4) and a voltage-sensing circuit (62) arranged to sense the voltage
supplied to the RF output stage by the charge-storing element; and
a pulsing circuit (64) coupled to the voltage sensing circuit (62) for pulsing the
or each power device (Q1 - Q4), the arrangement of the voltage sensing and pulsing
circuits being such that the timing of the pulses is controlled in response to the
sensed voltage.
7. A generator according to claim 6, characterised in that the voltage sensing circuit (62) and the pulsing circuit (64) are arranged to terminate
individual pulses of RF energy delivered by the RF power device or devices when the
sensed voltage falls below a predetermined level.
8. A generator according to claim 7, characterised in that the predetermined level is set such that the pulse termination occurs when the voltage
falls by a predetermined percentage value of between 5 percent and 20 percent.
9. A generator according to claim 6 or claim 7, characterised in that the predetermined level is set such that pulse termination occurs when the peak RF
voltage delivered at the output lines has fallen to a value of between 25V and 100V
below its starting value for the respective pulse.
10. A generator according to any of claims 6 to 9, characterised in that the power supply stage (50, 60, 62) and pulsing circuit (64) are arranged to generate
a pulsed RF output signal at the output terminals, which signal has a peak current
of at least 1A, a simultaneous peak voltage of at least 300 V, a modulation rate of
between 5Hz and 2kHz, and a pulse length of between 100µs and 5ms.
11. A generator according to claim 10, characterised in that the pulse length is between 0.5ms and 5ms.
12. A generator according to claim 10 or claim 11, characterised in that the pulse duty cycle is between 1% and 20%.
13. A generator according to any of claims 10 to 12, characterised in that the power supply stage (50, 60, 62) and pulsing circuit (64) are arranged to generate
a pulsed RF output signal at the output terminals (74), which signal has a peak voltage
of at least 300 V thoughout the entire pulse length.
14. A generator according to any of claims 10 to 13, characterised in that the power supply stage (50, 60, 62) and the pulsing circuit (64) are arranged to
generate, in an initial period, a pulsed r.f. output signal at the output terminals
(74), which signal has a peak current of at least 1A, a simultaneous peak voltage
of at least 300V, a modulation rate of between 5Hz and 2kHz, and a pulse length of
between 100µs and 5ms, and, in a subsequent period, to generate a constant power r.f.
output signal at the output terminals.
15. An electrosurgical generator according to any preceding claim, characterised in that the generator (10) is for supplying radio frequency (RF) power to an electrosurgical
instrument (12) for cutting or vaporising tissue in wet field electrosurgery, and
in that the output impedance of the output stage at the output lines (74) is less than 10
ohms.
16. An electrosurgical generator according to any of claims 1 to 14, characterised in that the generator (10) is for supplying radio frequency (RF) power to an electrosurgical
instrument (12) for cutting or vaporising tissue in dry field electrosurgery, and
in that the output impedance of the output stage at the output lines (74) is less than 50
ohms.
1. Elektrochirurgiegenerator (10) zur Versorgung eines Elektrochirurgieinstruments (12)
zum Schneiden oder Verdampfen von Gewebe mit Hochfrequenz(HF)-Energie, wobei der Generator
eine HF-Ausgangsstufe mit mindestens einer HF-Leistungseinheit (Q1-Q4), mindestens
ein Paar Ausgangsleitungen (74) zur Abgabe von HF-Leistung an das Instrument und einem
zwischen der HF-Leistungseinheit und dem Paar Ausgangsleitungen gekoppelten Ausgangs-Reihenschwingkreis
(L1, C1) umfasst, dadurch gekennzeichnet, dass die Ausgangsimpedanz der Ausgangsstufe an den Ausgangsleitungen kleiner als 200/√P
Ω ist, wobei P die maximale kontinuierliche HF-Ausgangsleistung des Generators in
Watt ist, und dass der Generator ferner eine Schutzschaltung (46, 66, 70, 72) umfasst,
die auf einen vorgegebenen elektrischen Zustand anspricht, der einen Ausgangs-Überlaststrom
anzeigt, um im wesentlichen die an die Ausgangsschaltung abgegebene HF-Leistung zu
unterbrechen.
2. Generator nach Anspruch 1, dadurch gekennzeichnet, dass die Schutzschaltung (46, 66, 70, 72) auf das Anlegen eines Kurzschlusses zwischen
den Ausgangsleitungen (74) anspricht und dass der Ausgangs-Reihenschwingkreis (L1,
C1) so gestaltet ist, dass die Anstiegsgeschwindigkeit des Ausgangsstroms an den Ausgangsleitungen
beim Anlegen des Kurzschlusses kleiner als (√P)/4 A/µs ist.
3. Generator nach Anspruch 1, dadurch gekennzeichnet, dass die Schutzschaltung (46, 66, 70, 72) auf das Anlegen eines Kurzschlusses an die Ausgangsleitungen
(74) anspricht, und dass die Schutzschaltung auf diesen Kurzschluss hinreichend schnell
anspricht, um die HF-Leistungseinheit (Q1-Q4) abzuschalten, bevor der durch diese
hindurchgehende Strom infolge des Kurzschlusses auf ein berechnetes Maximum ansteigt.
4. Generator nach Anspruch 3, dadurch gekennzeichnet, dass die Leistungseinheit (Q1-Q4) infolge des Anlegens eines Kurzschlusses an die Ausgangsleitungen
(74) abgeschaltet wird, wobei das Abschalten in einem Zeitraum erfolgt, der weniger
als 3 HF-Zyklen der abgegebenen HF-Leistung entspricht.
5. Generator nach einem der vorangehenden Ansprüche, dadurch gekennzeichnet, dass der vorgegebene elektrische Zustand einen Momentanstrom in der Ausgangsstufe anzeigt,
der einen vorbestimmten Wert überschreitet, und dass die Ansprechgeschwindigkeit der
Schutzschaltung (46, 66, 70, 72) so ist, dass dieser Zustand innerhalb desjenigen
HF-Zyklus detektiert wird, während dessen der Momentanstrom diesen Wert überschreitet.
6. Generator nach einem der vorangehenden Ansprüche,
gekennzeichnet durch
- eine mit der HF-Stufe gekoppelte Stromversorgungsstufe (50, 60, 62), die ein ladungsspeicherndes
Element (60) zur Abgabe von Energie an die Leistungseinheit(en) (Q1-Q4) und eine Spannungsfühlerschaltung
(62) umfasst, die so angeordnet ist, dass sie die vom ladungsspeichernden Element
an der HF-Ausgangsstufe abgegebene Spannung abtastet, und
- eine an die Spannungsfühlerschaltung (62) gekoppelte Impulsschaltung (64) zum Pulsen
der Leistungseinheit(en),
wobei die Anordnung der Spannungsfühlerschaltung und der Impulsschaltung so ist, dass
der Takt der Impulse abhängig von der abgetasteten Spannung gesteuert wird.
7. Generator nach Anspruch 6, dadurch gekennzeichnet, dass die Spannungsfühlerschaltung (62) und die Impulsschaltung (64) so angeordnet sind,
dass einzelne Impulse der von der/den HF-Leistungseinheit(en) abgegebene HF-Energie
abgebrochen werden, wenn die abgetastete Spannung unter einen vorbestimmten Wert fällt.
8. Generator nach Anspruch 7, dadurch gekennzeichnet, dass der vorbestimmte Wert ein solcher ist, dass der Impulsabbruch erfolgt, wenn die Spannung
um einen vorgegebenen Prozentwert zwischen 5 und 25% fällt.
9. Generator nach Anspruch 6 oder Anspruch 7, dadurch gekennzeichnet, dass der vorgegebene Wert so eingestellt ist, dass der Impulsabbruch erfolgt, wenn die
an die Ausgangsleitungen abgegebene HF-Spitzenspannung auf einen Wert zwischen 25
und 100 V unter den Anfangswert für den betreffenden Impuls gefallen ist.
10. Generator nach einem der Ansprüche 6 bis 9, dadurch gekennzeichnet, dass die Stromversorgungsstufe (50, 60, 62) und die Impulsschaltung (64) so angeordnet
sind, dass sie an den Ausgangsklemmen ein gepulstes HF-Ausgangssignal erzeugen, das
einen Spitzenstrom von mindestens 1 A, eine gleichzeitige Spitzenspannung von mindestens
300 V, eine Modulationsrate zwischen 5 Hz und 2 kHz und eine Impulsdauer zwischen
100 µs und 5 ms aufweist.
11. Generator nach Anspruch 10, dadurch gekennzeichnet, dass die Impulsdauer zwischen 0,5 und 5 ms beträgt.
12. Generator nach Anspruch 10 oder Anspruch 11, dadurch gekennzeichnet, dass das Taktverhältnis der Impulse zwischen 1 und 20% beträgt.
13. Generator nach einem der Ansprüche 10 bis 12, dadurch gekennzeichnet, dass die Stromversorgungsstufe (50, 60, 62) und die Impulsschaltung (64) so angeordnet
sind, dass sie an den Ausgangsklemmen (74) ein gepulstes HF-Ausgangssignal erzeugen,
das während der gesamten Impulsdauer eine Spitzenspannung von mindestens 300 V hat.
14. Generator nach einem der Ansprüche 10 bis 13, dadurch gekennzeichnet, dass die Stromversorgungsstufe (50, 60, 62) und die Impulsschaltung (64) so angeordnet
sind, dass sie während eines ersten Zeitintervalls an den Ausgangsklemmen (74) ein
gepulstes HF-Ausgangssignal erzeugen, das einen Spitzenstrom von mindestens 1 A, eine
gleichzeitige Spitzenspannung von mindestens 300 V, eine Modulationsrate zwischen
5 Hz und 2 kHz und eine Impulsdauer zwischen 100 µs und 5 ms aufweist, und dass sie
in einem folgenden Zeitintervall an den Ausgangsklemmen ein HF-Ausgangssignal konstanter
Leistung erzeugen.
15. Elektrochirurgiegenerator nach einem der vorangehenden Ansprüche, dadurch gekennzeichnet, dass der Generator (10) zur Abgabe von Hochfrequenz (HF)-Leistung an ein Elektrochirurgieinstrument
(12) zum Schneiden und Verdampfen von Gewebe bei der Elektrochirurgie im nassen Umfeld
bestimmt ist, und dass die Ausgangsimpedanz der Ausgangsstufe an den Ausgangsleitungen
(74) kleiner als 10 Ω ist.
16. Elektrochirurgiegenerator nach einem der Ansprüche 1 bis 14, dadurch gekennzeichnet, dass der Generator (10) zur Abgabe von Hochfrequenz(HF)-Leistung an ein Elektrochirurgieinstrument
(12) zum Schneiden oder Verdampfen von Gewebe bei der Elektrochirurgie im trockenen
Umfeld bestimmt ist, und dass die Ausgangsimpedanz der Ausgangsstufe an den Ausgangsleitungen
(74) kleiner als 50 Ω ist.
1. Générateur électrochirurgical (10) propre à délivrer une puissance radiofréquence
(RF) à un instrument électrochirurgical (12) pour le découpage ou la vaporisation
d'un tissu, dans lequel le générateur comprend un étage de sortie RF ayant : au moins
un dispositif de puissance RF (Q1 - Q4), au moins une paire de lignes de sortie (74)
servant à fournir la puissance RF à l'instrument, et un réseau de sortie résonnant
série (L1, C1) couplé entre le dispositif de puissance RF et ladite paire de lignes
de sortie, caractérisé en ce que l'impédance de sortie de l'étage de sortie sur les lignes de sortie est inférieure
à 200/√P ohms, où P est la puissance de sortie RF continue maximum du générateur en watts, et en ce que le générateur comprend en outre un circuit de protection (46, 66, 70, 72) sensible
à un état électrique prédéterminé indicatif d'une surcharge de courant de sortie pour
interrompre sensiblement la puissance RF fournie au réseau de sortie.
2. Générateur selon la revendication 1, caractérisé par un circuit de protection (46, 66, 70, 72) sensible à l'application d'un court-circuit
entre les lignes de sortie (74), et caractérisé en ce que le réseau de sortie résonnant série (L1, C1) est tel que la vitesse de montée du
courant de sortie sur les lignes de sortie, lorsque le court-circuit est appliqué,
est inférieure à (√P)/4 ampères par microseconde.
3. Générateur selon la revendication 1, caractérisé par un circuit de protection (46, 66, 70, 72) sensible à l'application d'un court-circuit
entre les lignes de sortie (74), et caractérisé en ce que le circuit de protection est sensible audit court-circuit suffisamment rapidement
pour mettre hors service le dispositif de puissance RF (Q1 - Q4) avant que le courant
le traversant n'augmente jusqu'à atteindre un courant maximum nominal en raison du
court-circuit.
4. Générateur selon la revendication 3, caractérisé en ce que le dispositif de puissance (Q1 - Q4) est mis hors service en réponse à l'application
du court-circuit sur les lignes de sortie (74), la mise hors service se produisant
dans une période de temps correspondant à moins de 3 cycles RF de la puissance RF
délivrée.
5. Générateur selon l'une quelconque des revendications précédentes, caractérisé en ce que l'état électrique prédéterminé est indicatif d'un courant instantané dans l'étage
de sortie dépassant un niveau prédéterminé, et en ce que la rapidité de réponse du circuit de protection (46, 66, 70, 72) est telle que ledit
état est détecté dans le cycle RF pendant lequel le courant instantané dépasse ledit
niveau.
6. Générateur selon l'une quelconque des revendications précédentes, caractérisé par : un étage d'alimentation électrique (50, 60, 62) couplé à l'étage de sortie RF,
l'étage d'alimentation électrique comprenant un élément de stockage de charge (60)
servant à fournir une puissance au dispositif ou aux dispositifs de puissance (Q1
- Q4) et un circuit de détection de tension (62) configuré pour détecter la tension
fournie à l'étage de sortie RF par l'élément de stockage de charge ; et
un circuit d'émission d'impulsions (64) couplé au circuit de détection de tension
(62) propre à envoyer des impulsions au ou à chaque dispositif de puissance (QI -
Q4), la configuration des circuits de détection de tension et d'émission d'impulsions
étant telle que la distribution des impulsions est contrôlée en réponse à la tension
détectée.
7. Générateur selon la revendication 6, caractérisé en ce que le circuit de détection de tension (62) et le circuit d'émission d'impulsions (64)
sont configurés pour interrompre les impulsions individuelles d'énergie RF fournies
par le dispositif ou les dispositifs de puissance RF quand la tension détectée tombe
au-dessous d'un niveau prédéterminé.
8. Générateur selon la revendication 7, caractérisé en ce que le niveau prédéterminé est fixé de telle sorte que l'arrêt des impulsions se produit
quand la tension tombe d'un pourcentage prédéterminé compris entre 5 pour cent et
20 pour cent.
9. Générateur selon la revendication 6 ou la revendication 7, caractérisé en ce que le niveau prédéterminé est fixé de telle sorte que l'arrêt des impulsions se produit
quand la tension RF de crête fournie aux lignes de sortie est tombée jusqu'à une valeur
comprise entre 25 V et 100 V au-dessous de sa valeur initiale pour l'impulsion respective.
10. Générateur selon l'une quelconque des revendications 6 à 9, caractérisé en ce que l'étage d'alimentation électrique (50, 60, 62) et le circuit d'émission d'impulsions
(64) sont agencés pour générer un signal de sortie RF pulsé sur les bornes de sortie,
lequel signal possède un courant de crête d'au moins 1 A, une tension de crête simultanée
d'au moins 300 V, un taux de modulation compris entre 5 Hz et 2 kHz et une longueur
d'impulsion comprise entre 100 µs et 5 ms.
11. Générateur selon la revendication 10, caractérisé en ce que la longueur d'impulsion est comprise entre 0,5 ms et 5 ms.
12. Générateur selon la revendication 10 ou la revendication 11, caractérisé en ce que le coefficient d'utilisation des impulsions est compris entre 1 % et 20 %.
13. Générateur selon l'une quelconque des revendications 10 à 12, caractérisé en ce que l'étage d'alimentation électrique (50, 60, 62) et le circuit d'émission d'impulsions
(64) sont agencés pour générer un signal de sortie RF pulsé sur les bornes de sortie
(74), lequel signal possède une tension dé crête d'au moins 300 V sur toute la longueur
d'impulsion.
14. Générateur selon l'une quelconque des revendications 10 à 13, caractérisé en ce que l'étage d'alimentation électrique (50, 60, 62) et le circuit d'émission d'impulsions
(64) sont agencés pour générer, dans une période initiale, un signal de sortie r.f.
pulsé sur les bornes de sortie (74), lequel signal possède un courant de crête d'au
moins 1 A, une tension de crête simultanée d'au moins 300 V, un taux de modulation
compris entre 5 Hz et 2 kHz, et une longueur d'impulsion comprise entre 100 µs et
5 ms, et, dans une période suivante, pour générer un signal de sortie r.f. de puissance
constante sur les bornes de sortie.
15. Générateur électrochirurgical selon l'une quelconque des revendications précédentes,
caractérisé en ce que le générateur (10) est propre à délivrer une puissance radioélectrique (RF) à un
instrument électrochirurgical (12) pour le découpage ou la vaporisation d'un tissu
au cours d'une électrochirurgie en champ opératoire humide, et en ce que l'impédance de sortie de l'étage de sortie sur les lignes de sortie (74) est inférieure
à10 ohms.
16. Générateur électrochirurgical selon l'une quelconque des revendications 1 à 14, caractérisé en ce que le générateur (10) est propre à délivrer une puissance radioélectrique (RF) à un
instrument électrochirurgical (12) pour le découpage ou la vaporisation d'un tissu
au cours d'une électrochirurgie en champ opératoire sec, et en ce que l'impédance de sortie de l'étage de sortie sur les lignes de sortie (74) est inférieure
à 50 ohms.